Commercial applications for battery-powered, radio frequency modems continue to expand at a substantial rate. Consumer demand for such battery-powered modems results in part from the propagation of battery powered computers and the increased availability of cellular telephone services. The propagation of battery-powered personal computers has resulted in dramatic changes with respect to where, when, and how computer related work projects can be done. Similarly, the growth of the cellular telephone industry has freed users telephone from the constraints of fixed site facilities. Battery-powered, radio frequency modems constitute another substantial technical step in the development of a portable, computer work stations that can be in voice and/or data communication with facilities anywhere in the world.
One of the well-known constraints in the operation of portable computers and cellular telephones relates to the expected life of the battery used to power the device. Once battery power is lost, the device becomes useless until recharged or connected to another power source. Consequently, users of such devices commonly carry extra batteries and/or purchase expensive long life batteries. Portable, radio-frequency modems face similar operating constraints resulting from power requirements. Consequently, a need has arisen to develop techniques for preserving battery life as much as possible.
Power conservation techniques are limited by several interrelated factors. Principal among those factors are cost, power consumption related to the conservation techniques themselves, and the time during which power may be conserved. Ideally, power consumption would be provided by a low-cost circuit, which itself consumes little energy, and can be left in a stand-by or off condition except during limited operational periods. Conventional power conservation circuits have significant limitations in their ability to achieve that combination of goals. For example, crystal oscillator circuits provide high precision, which allows the stand-by time to be minimized. However, crystal oscillator circuits suffer from drawbacks associated with both the cost and increased power consumption when in the stand-by condition.
By comparison, RC oscillator circuits are substantially less expensive than crystal oscillator circuits, and consume less power during stand-by condition. However, RC oscillator circuits do not typically achieve the precision available from crystal oscillator circuits. Consequently, RC oscillator circuits need to be in a powered condition for a longer period to assure the power is available the precise time that it is required.
The present invention is directed to apparatus and technique for optimizing power conservation in a low-cost RC oscillator circuit. The present invention takes advantage of the low cost and low power consumption of RC oscillator circuits, while providing a technique for enhancing the precision of the RC oscillator circuit by adaptive calibration to a network timing signal. The adaptive calibration allows adjustment of the stand by or "sleep" cycle of the modem to accommodate static and/or dynamic delays or inaccuracies in the RC oscillator, power circuitry, and network timing. The invention, therefore, permits the implementation of a low-cost, radio-frequency modem having extended battery life and reliable operation.